Permanent magnet synchronous motors (PMSMs) are utilized in various applications because they have generally favorable efficiency characteristics relative to other types of motors. Typically, PMSMs have three separate electrical windings within the stator which are each powered by an oscillating alternating current (AC) voltage source. The shaft torque of the motor and the power conversion efficiency depend upon both the magnitude and the phase angle of the oscillating voltage.
In certain applications, such as electric vehicles and hybrid electric vehicles, electrical power is available from a non-oscillating direct current (DC) voltage source such as a battery. Therefore, inverters are utilized to convert the non-oscillating voltage into three oscillating voltages. Inverters contain a discrete number of switching devices and are therefore capable of supplying only a discrete number of voltage levels at each of the three motor terminals. For a 2-level inverter, at any moment in time, the switching devices are set to electrically connect each of the three AC terminals to either the positive or the negative DC terminal. Thus, eight switching states are available. Two of these switching states, in which all three terminals are connected to the same DC terminal, are called zero states. In the remaining six states, one AC terminal is connected to one of the DC bus terminals and the other two AC terminals are connected to the opposite DC bus terminal. The inverter is capable of switching rapidly among these eight states.
Two basic control methods are known for switching among inverter states to regulate torque output of a PSMS. In the six-step method, the inverter cycles through the six non-zero states once per cycle of the rotor, producing an oscillating voltage and current in each winding. A rotor cycle is defined relative to motor poles and does not necessarily correspond to a complete revolution. The amplitude of the AC voltage is dictated by the DC voltage. The torque is dictated by the DC voltage, the rotor speed, and the phase difference between these quasi-sinusoidal AC voltage signals and the rotor position. A controller issues commands to the inverter indicating when to switch to the next state in the sequence. In the PWM method, the inverter switches very rapidly among two of the non-zero states and one of the zero states. A controller specifies what fraction of the time should be spent in each of these three states by specifying PWM duty cycles. The controller updates these duty cycles at regular intervals such that the frequency of updates is significantly higher than the frequency of the rotor rotation.
Some general characteristics of typical PMSMs are illustrated in an exemplary embodiment shown in FIG. 1. The operating region depends upon the DC voltage. The positive speed, positive torque operating region at a reference DC voltage may be bounded as illustrated by solid lines 110, 112, and 114. At low speeds, the maximum available torque may be limited by a maximum winding current as indicated by line 110. Line 112 indicates a maximum available torque at higher speeds which is limited by the voltage. At point 116, called the corner point, both current and voltage are at their respective maximums. Line 114 indicates an overall maximum rated speed. The dotted lines indicate the corresponding operating region at a higher DC voltage above the reference DC voltage. The PMSM cannot realize bursts of torque exceeding the maximum winding current without saturating the magnetic core and rendering any increase in current or voltage as useless. The PMSM can be irreparably damaged by excessive sparking at the commutator if a burst of torque exceeds the maximum operating torque rating of the electric machine.
PMSMs may generate either positive or negative torque and may rotate in either positive or negative directions. In the positive speed, negative torque quadrant, a PMSM acts as a generator converting mechanical energy into electrical energy. In this quadrant, the characteristics are similar to that shown in FIG. 1, although the minimum torque curve corresponding to the voltage limit may not be a mirror image of line 112. The negative speed region closely tracks the positive speed region rotated 180 degrees about the origin.